Workers at the Jet Propulsion Laboratory in Pasadena, Ca. have developed a method for measuring ocean surface currents employing a synthetic aperture radar (SAR) of a type well known to those skilled in the art. In the interest of simplicity and the avoidance of redundancy, the operation of SAR will not be discussed herein as it is of no relevance to the present invention. As depicted in FIG. 1, the method involves differencing images of surface waves formed by two antennas 10, 10' on an aircraft 12. Their approach uses a conventional SAR system with the two antennas 10, 10' mounted fore and aft along the fuselage of the aircraft 12. One antenna 10 transmits and the signals received at both antennas 10, 10' are first processed separately into two images by the SAR 14. The data of the two images are stored in the memory 16 and then combined interferometrically at a later time by appropriate logic at a ground site.
A fundamental problem with this method is that it assumes the motion of the aircraft 12 to be parallel to the radar baseline (i.e. the vector separation of the antennas 10, 10'). If the aircraft 12 drifts sideways, the radar will interpret the motion as a radial velocity of the water below. For example, in one system which was investigated, an error of 0.01 degree in the azimuth of the SAR baseline will induce an error of 4 cm/s in the velocity of the current along the line of sight. Until now, this problem has been dealt with by taking care to include a stationary area (i.e. land) in the radar images and using this filed region to calibrate both the velocity and orientation of the aircraft 12. Obviously, the need for this calibration limits the application of the technique to areas near land.